What is the fundamental character of light?

Light is transverse and electromagnetic radiation that exhibits the properties of a wave as well as a particle. For this reason, light is said to have a “dual nature.”

What is light?

Light or visible light is a natural agent that stimulates sight and makes things visible. It shines on an object, bounces off or reflects to our eyes. It is intimately involved in our daily lives and is a source of illumination, such as sunlight or artificial light (lamps or luminaries). Some of the characteristics of light are listed below:

• Light can make 7.5 laps around the Earth in one second.
• It takes light an average of 8 minutes 20 seconds to reach the surface of the earth from the sun.
• The speed of light is universal in all inertial frames of reference.
• The speed of light in vacuum is fixed, i.e., $3\times {10}^8 {\mathrm{ms}}^{-1}$.
• The speed of light in a medium is less than that in vacuum.
• The speed of light is dependent on the medium through which it propagates.

Scientifically, light can be defined as a form of electromagnetic (EM) radiation or waves that are all around us. Electromagnetic radiations and matter are the two essential components of the universe. The electromagnetic spectrum is quite broad ranging from shorter wavelengths (gamma and X-rays) to longer wavelengths (micro and radio waves).

Is light a wave or a particle?

What is true about the nature of light? Is it described as a wave or just a flow of particles? Such type questions have perplexed scientists for long. Known for his famous universal law of gravitation, Sir Isaac Newton (sometime around 1700) realized that light and frequency have quite similar properties. He used a glass prism to observe the dispersion of visible light into its component colors. He came to a conclusion that light is a group of particles, based on his observation that the boundary of the shadows created by light is very clear and sharp. This is known as the corpuscular theory.

Around the same time, a Dutch physicist, Christian Huygens founded the wave theory of light and put forward Huygen’s principle. His theory served as a fundamental explanation for phenomena like reflection, refraction, diffraction and interference of visible light. According to this principle, diffraction is the bending of a wave around the edges of any obstacle. For example, if a pebble is thrown into a pond, then it will create ripples that are circular in nature. These ripple-like formations are termed wavefronts.

In 1861, the third theory of light was proposed by a physicist J.C. Maxwell where he speculated the existence of electromagnetic waves and upheld the wave theory of light. He presented four fundamental equations for electromagnetic theory, known as Maxwell’s equations, which showed the inter-relation between the electric and magnetic fields.

The particle theory of light vanished completely until the 19^th century. However, Sir Albert Einstein revived it in the late 19^th century. Till now, light was proved to be “a particle as well a wave” and its dual nature emerged from the electromagnetic theory to quantum mechanics. According to Einstein’s theory, light is made up of photons that are invisible to the naked eye and the directed flow of photons is a wave. The main point of this theory is that the incident energy of light (photons) depends on the oscillation frequency of the photons.

Einstein conducted research on the photoelectric effect, a phenomenon where light with energy above a certain threshold hits a metal surface and a surface electron gets emitted. A strange fact about this phenomenon is that the energy of the emitted electrons, known as photoelectrons, is independent of the energy of the incident light or radiation.

Photons

A photon is a fundamental particle of light defined as a discrete bundle of energy called “quanta.” Max Planck introduced the concept of light quanta in the year 1900, which became a crucial component in the discovery of a new class of fundamental particles called quantum particles. According to a more chemistry-oriented picture of a photon, it is the energy released when an electron transits to a lower level of energy in an atom. We know that electrons are present only in specific orbitals or energy levels. So, any photon can be a result of a falling electron.

The energy, E of a photon is given as,

$E=h\nu$

Where, $h=6.63\times {10}^{-34} \mathrm{Js}$ is the Planck’s constant and v is the frequency of the photon.

Or

$E=\frac{hc}{\lambda }$

Where, $c=3\times {10}^8 {\mathrm{ms}}^{-1}$ is the speed of light and $\lambda$ is the wavelength of the photons.

Some key properties of photons are listed below:

• Photons travel in a straight line with a constant speed, i.e., the speed of light in free space.
• Photons are electrically neutral and stable with zero rest mass.
• They carry energy as well as momentum, which are frequency-dependent.
• A photon can have a particle-like interaction with an electron and other subatomic particles.
• Photons do not decay unexpectedly in empty space because they do not have any smaller subatomic particles.
• A photon can be destroyed or created when radiation is absorbed or emitted.
• They are not deviated by electric or magnetic fields.

Light reflects

How is it possible for us to see a distant tree sometimes so clearly on the surface of a pond or a lake?

The light from the sun strikes a tree and bounces back in different directions. This is known as the reflection of light. If the surface of the water body is calm with no wind, then the angle of incident light (angle of incidence, i) is equal to the angle of the light that bounces back from the surface (angle of reflection, r). This is called spectacular or mirror reflection. The reflected part of the light reaches our eyes and converges it on the retina through the lens. Thus, we can see a clear image of the tree on the surface of a pond. For simplification, an illustration is given below that shows a person looking at a distant tree.

Light scatters

Why does the color of the blue sky changes to red in the evening?

The sunlight travels through space and reaches the earth. It disperses after striking several suspended particles of dust in the atmosphere. A component of this visible light returns to the outer space and the remaining part reaches us on the earth. This is known as Rayleigh scattering. The intensity of the scattered light, I is inversely proportional to the fourth power of the wavelength, λ. Mathematically,

$I\propto \frac{1}{{\lambda }^4}$

The size of the dust particles is very small as compared to the wavelength of the incident light and they do not scatter all the colors equally. The color blue has a much shorter wavelength; therefore, it scatters more strongly in all directions, creating an effect that we see as the blue sky. On the contrary, the color red has the largest wavelength. During sunset, we see a red or an orange sky because the blue light has already scattered out. So, the light received by the observer is mostly of a longer wavelength.

Light refracts

Why does a straw placed in a glass full of water appear to be bent or broken?

Light rays get refracted at the interface of any two media; here, air and water. The phenomenon of refraction takes place because speed of light varies in air and water. Refraction occurs when the light from the water is directed towards the air. However, this light coming out of the water appears to move along the line of sight toward us. Our eyes form a false image on the line extending from the refracted ray. Thus, the bottom part of the straw in the water seems to be deviated from its actual position.

Common Mistakes

It should be noted that there is nothing in this universe that can travel faster than light. Not even the fastest rocket or the most accelerated atomic particle. The speed of light is referred to as the “speed limit of the universe”.

Students often get confused that all elements show a photoelectric effect. However, it should be made clear that nonmetals do not show a photoelectric effect. Only metals and alkali metals are capable of ejecting photoelectrons. It should be noted that even liquids and gases exhibit this effect but to a limited extent.

Formulas

• Energy of a photon, E is given as, $E=h\nu =\frac{hc}{\lambda }$ where $h=6.63\times {10}^{-34} \mathrm{Js}$ is the Planck’s constant, ν is the frequency of the photons, $c=3\times {10}^8 {\mathrm{ms}}^{-1}$ is the speed of light and λ is the wavelength of incident photons.
• According to Rayleigh scattering the intensity of scattered light, I is given as $I\propto \frac{1}{{\lambda }^4}$ where λ is the wavelength of the incident photons.

Context and Applications

This topic is significant in the professional exams for undergraduate and postgraduate courses especially for:

Bachelors of Science in Physics

Masters of Science in Physics

Bachelor of Engineering in Electrical Engineering

Bachelor of Engineering in Mechanical Engineering

Master of Science in Geoinformatics

Master of Science in Advanced Quantum Mechanics

Related Concepts

Electromagnetic Spectrum

Quantization of Energy

Reflection

Refraction

Diffraction

Practice Problems

Q1: In simpler terms, reflection of light can be defined as,

1. Bouncing back of light
2. Absorption of light
3. Passing through of light
4. Both (a) and (b)

Correct option: (a)

Q2: Which of the following is a natural source of light?

1. Light bulb
2. Neon Bulb
3. Sun
4. Television

Correct option: (c)

Q3: Which of the fooling phenomena proved the particle nature of light?

1. Interference
2. Diffraction
3. Polarization
4. Photoelectric effect

Correct option: (d)

Q4: If the wavelength of the incident light waves is decreased, the energy of the light will,

1. Decrease
2. Increase
3. Remains unchanged
4. None of the above

Correct option: (b)

Q5: The speed of light in vacuum is:

1. $3\times {10}^8 {\mathrm{ms}}^{-1}$
2. $3\times {10}^{10} {\mathrm{ms}}^{-1}$
3. $1.5\times {10}^8 {\mathrm{ms}}^{-1}$
4. $2.5\times {10}^8 {\mathrm{ms}}^{-1}$

Correct answer: (a)